1. Field of the Invention
This invention relates to a position reader having a display device and an input means, and more particularly to a position reader having a display device such as a liquid-crystal display device, provided with a tablet.
2. Description of Related Art
In recent years, portable terminal units have employed liquid-crystal display devices as display means, where a tablet making use of resistance thin films is provided on a screen as an input means, and the surface of the tablet is pushed down with a pen, a finger or the like to perform input. As the prior art of a means for accurately reading this pen-down position on the tablet, for example, Japanese Patent Application Laid-open (KOKAI) No. 5-150902 discloses an example in which an STN (Super Twisted Nematic) type device is used as the liquid-crystal display device.
In the position reader having such a display device provided with a tablet on its screen, the tablet is constituted of a horizontal-direction transparent resistance film formed of a transparent resistance film and respectively provided with horizontal electrodes at opposing two sides in the direction of a horizontal coordinate axis, and a vertical-direction transparent resistance film also formed of a transparent resistance film and respectively provided with vertical electrodes at opposing two sides in the direction of a vertical coordinate axis, both films being put together to leave a given minute gap between them, and such a tablet is provided on the screen of the display device.
With such a constitution, an electric current flows to the horizontal-direction transparent resistance film extending between the two horizontal electrodes, and an electric current also flows to the vertical-direction transparent resistance film extending between the two vertical electrodes. The voltages across these electrodes (herein often "interelectrode voltages") are respectively detected, and separately fed to A/D (analog-to-digital) converters, where the signals are converted to digital data and fed to a CPU (central processing unit).
Now, assume that the surface of the tablet has been pushed down with a pen at an arbitrary position, the voltage across the two horizontal electrodes changes in accordance with the pen-down position in the horizontal axis direction on the tablet, and also the voltage across the two vertical electrodes changes in accordance with the pen-down position in the vertical axis direction on the tablet. The changes of these voltages are detected by the CPU on the basis of the digital data fed from the A/D converters, and a horizontal coordinate point and a vertical coordinate point at the pen-down position on the tablet are determined. Then, the CPU controls a display controller on the basis of the information of the horizontal and vertical coordinate points, so that picture elements at the position corresponding to the pen-down position of the tablet on the screen of the display device are driven to display the pen-down position.
FIG. 16, reference symbol (a), illustrates a screen of the display device. Its display area 101 is comprised of lines each formed of X dots in the horizontal direction, forming Y lines of lines Y0, Y1, . . . , Ymax arranged in the vertical direction. In order to display a picture in such a display area, a frame signal 301 and a horizontal scanning signal 105 (these signals and transfer signals of display data are hereinafter called display drive signals collectively) are fed to the display device.
This frame signal 301 is level-reversed for each frame period that is necessary for scanning the whole display area 101. Hence, in this case, the voltage of liquid-crystal display drive signals is alternated in one frame. The horizontal scanning signal 105 is also a pulse signal for successively driving the lines Y0, Y1, . . . , Ymax of the display area 101, and this enables all the lines Y0, Y1, . . . , Ymax to be successively driven in one frame period between the point in time of the level reversal of the frame signal 301 and the point in time of the next level reversal. Then, the display data of the whole dots of these lines are simultaneously fed to the lines Yi (i=1, 2, . . . , max) thus driven.
Now, in the position reader constituted as described above, electrostatic coupling is formed between the screen of the display device and the tablet. Such electrostatic coupling causes, as shown in FIG. 16, reference symbol (c), noise ascribable to the above display drive signals of the display device. More specifically, when the frame signal 301 and the horizontal scanning signal 105 are level-reversed, noise occurs concurrently therewith in the signals fed from the tablet, which occurs through the electrostatic coupling. This noise is significant especially at the point where the frame signal 301 changes, and such noise increases with an increase in the tablet area that becomes larger as the display device is made larger.
Once such noise has occurred, the noise disturbs the interelectrode voltages of the tablet, and the voltages thus disturbed are fed to the A/D converters, so that the CPU may perform erroneous positional detection.
Accordingly, in the art disclosed in the above Japanese Patent Application Laid-open (KOKAI) No. 5-150902, in order to prevent this noise so as to enable positional detection with good precision, the A/D converters are so designed as to accept the interelectrode voltages at the time when the level has become stable other than the timing at which the frame signal 301 and the horizontal scanning signal 105 are level-reversed.
Stated more specifically, the horizontal scanning signal 105 that drives the lines Y0, Y1, . . . , Ymax shown in FIG. 16, reference symbol (b), includes a plurality of pulses each having equal pulse widths which are outputted as Y0, Y1, . . . , Ymax at the falling edges of the respective signals. The frame signal 301 is formed by, e.g., counting the clock pulses of the signals. Since the levels of the frame signal 301 and horizontal scanning signal 105 are stable at positions posterior to the rising edges of clock pulses, the A/D converters are so designed as to utilize the rising edge of clock pulses to accept the interelectrode voltages of the tablet.
Thus, in order to read the output of the tablet at the timing where the levels of the frame signal 301 and horizontal scanning signal 105 are stable, the CPU first applies a voltage and, after the applied voltage has become stable, detects whether or not the tablet is in the state of pen-down. Thereafter, it applies a voltage to the tablet once more and, after this applied voltage has become stable, it detects the rising edges of the clock pulses and utilizes the rising edges of clock pulses to accept the output of the tablet through the A/D converters to detect the pen-down position of the tablet.
In the prior art as described above, in order to prevent noise from the liquid-crystal screen to the tablet, the CPU utilizes the rising edge of the horizontal scanning signal of the liquid-crystal screen to read the coordinate data while converting the output voltages of the tablet into digital form by means of the A/D converters. Hence, as a problem, the CPU must stop the processing of reading the coordinate data until the rising edges appear in the horizontal scanning signal 105 of the liquid-crystal screen even after the applied voltages fed to the tablet have become stable. It has not been taken into consideration to appropriate the CPU for other processing during the stop so that its throughput can be effectively utilized.
As another problem, without regard to whether or not the tablet is in the state of pen-down, the voltages are respectively, periodically, applied across the horizontal electrodes and across the vertical electrodes to cause electric currents to flow the horizontal and vertical transparent resistance films of the tablet so as to detect that the tablet is in the state of pen-down. Thus, it has not been taken into consideration to save the time and power consumption required for the CPU to detect that the tablet is in the state of pen-down.
As still another problem, immediately before the pen comes away from the surface of the tablet from a pen-down state (hereinafter called "pen-up motion"), the horizontal and vertical transparent resistance films respectively formed on the top surface and bottom surface of the tablet may come into insufficient contact or non-contact with each other, resulting in a significant lowering of the reliability of the coordinate data obtained at that point in time. This problem, however, has not been taken into consideration at all in the prior art described above.